In a well completion, a string of casing or pipe is set in a well bore and cement is forced into the annulus between the casing and the well bore, primarily to separate oil and gas producing horizons from each other and from water-bearing strata.
If the cement fails to provide a separation of one zone from another, then fluids under pressure from one zone may be able to migrate and contaminate an otherwise productive nearby zone. Migration of water in particular produces undesirable water cutting of a producing zone and possibly can make a well noncommercial.
Cement failures can occur in a variety of manners. For example, there may, for one reason or another, be a complete absence of cement behind the casing segment where the cement should be. This would be a gross cement bonding failure leading to rapid contamination between zones intended to be separated.
Another type of cement failure arises when the cement is present behind the casing, but a small cement-free annulus exists between the cement and casing. This annulus may be so thick as to enable hydraulic communication between zones leading to undesirable contamination.
Such annulus, however, may also be so thin as to effectively preserve the hydraulic security function of the cement. Such acceptable small annulus may arise from the technique employed to introduce the cement in the first place. For example, the cement typically is introduced under very high pressure such as produced by using a heavy mud to chase the cement plug down and into the annulus around the casing. The resulting pressure inside the casing causes a slight expansion of the casing and subsequent contraction when the heavy mud is removed. The magnitude of the contraction depends upon the pressure and casing thickness and tends to result in a slight separation, an annulus, between the cement and casing. It is important to know whether the cement is performing its function, i.e. whether the cement bond is hydraulically secure.
Techniques have been proposed to ascertain the quality of the cement bond. In this sense the term "bond" as used herein, is to be understood to include both those cases where the cement actually adheres to the casing as well as when there is no adhesion but instead a small micro-annulus which is so small as to prevent fluid communication between cement separated zones. In other words, the term "good bond" means that separation of zones by the cement is adequate to prevent fluid migration between the zones even in the presence of a micro-annulus. It is, therefore, desirable that cement evaluation techniques identify such micro-annuli as good cement bonds while recognizing annuli incapable of separating zones as hydraulically insecure or bad bonds.
When an acoustic pulse producer such as described in Norel et al is employed in an ultrasonic echo testing device as described in Russian Patent SU No. 405095 or the U.S. Pat. No. 3,974,476 to Cowles, the increased spacing suggested by the latter between the transducer and the casing enables reception of a greater number of cycles. However, in such case the intermediate layers proposed by Norel et al between the transducer and the casing tend to severely attenuate the echo signals which already arrive with reduced amplitude by virtue of the increased spacing.